Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer raceways and a plurality of rolling elements (balls or rollers) disposed therebetween. For long-term reliability and performance it is important that the various elements have a high resistance to rolling contact fatigue, wear and creep. The bearing components are typically manufactured from a bearing steel.
Titanium and its alloys exhibit a low density relative to many other structural metals and alloys, excellent corrosion resistance and high specific proof strengths (strength/density). At 885° C., pure titanium undergoes an allotrophic change from a hexagonal close packed structure to a body-centered cubic structure. Alloying elements are classified by whether they stabilise the α (low temperature, hcp) or β (high temperature, bcc) crystal forms. α-stabilisers, such as aluminium and tin, are typically added in order to increase the creep resistance of a titanium alloy. In contrast, β-stabilisers, such as vanadium, molybdenum and zirconium, are typically added to increase the strength.
The alloy known as VT22 comprises from 0.5 to 1.5 wt % Fe, from 0.5 to 2 wt % Cr, from 4 to 5.5 wt % Mo, from 4 to 5.5 wt % V, from 4.4 to 5.9 wt % Al, at most 0.1 wt % C, at most 0.15 wt % Si, at most 0.05 wt % N, at most 0.3 wt % Zr, at most 0.2 wt % O, at most 0.015 wt % H and the balance Ti together with unavoidable impurities.
Notwithstanding their advantageous properties, conventional titanium alloys, such as VT22, exhibit lower hardness, strength and impact toughness compared to bearing steel. Furthermore, titanium alloys have the tendency to gall even in the presence of good lubrication.
In order to overcome such a problem, wear-resistant coatings have been applied. In addition, in bearing applications, non-metallic counter faces have been used, such as silicon nitride balls coated with diamond-like carbon. However, there are currently no titanium alloys that exhibit sufficient resistance to surface fatigue and rolling contact fatigue such that they could be used in bearing components without having to use the above measures.
JP 11153140 describes a titanium alloy comprising from 1.0 to 5.0 wt % Cr and the balance Ti. After α′ martensite quenching this alloy has high hardness and is used for manufacturing bearing rings. However, this alloy has a low thermal stability due to the presence of 80% metastable α′-martensite, which is problematic since bearings often experience heating during use.
US 2004/0231756 describes a titanium alloy comprising from 3.2 to 4.2 wt % Al, from 1.7 to 2.3 wt % Sn, from 2.0 to 2.6 wt % Zr, from 2.9 to 3.5 wt % Cr, from 2.3 to 2.9 wt % Mo, from 2.0 to 2.6 wt % V, from 0.25 to 0.75 wt % Fe, from 0.01 to 0.08 Si, 0.21 wt % or less O, and the balance Ti. After heat treatment this alloy exhibits both high strength and high ductility. However, since the ductility is so high, the alloy is unable to withstand high contact stresses.
It is an objective of the present invention to address or at least mitigate some of the problems associated with prior art and to provide a method of heat-treating a titanium alloy which can be used in the manufacture of a bearing component.